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<title>lazy&lt;T, Executor = void&gt;/atomic_lazy&lt;T, Executor = void&gt; - Boost.Outcome documentation</title>
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  <div class="titlepage"><div><div><h1 style="clear: both"><code>lazy&lt;T, Executor = void&gt;/atomic_lazy&lt;T, Executor = void&gt;</code></h1></div></div></div>
  <p>This is very similar to <a href="../../../reference/types/awaitables/eager.html" class="api-reference"><code>eager&lt;T, Executor = void&gt;</code></a>
, except that execution of the
<code>lazy&lt;T&gt;</code> returning function suspends immediately. Functions which return <code>lazy&lt;T&gt;</code>
are therefore suitable for tasks which you need to instantiate right now, but whose
execution will occur elsewhere e.g. in a separate kernel thread. Because of the very
common use case of using worker threads to execute the body of lazily executed
coroutines, most people will want to use <code>atomic_lazy&lt;T&gt;</code> instead of <code>lazy&lt;T&gt;</code>.</p>

<p><code>atomic_lazy&lt;T&gt;</code> is like <code>lazy&lt;T&gt;</code>, except that the setting of the coroutine result
performs an atomic release, whilst the checking of whether the coroutine has finished
is an atomic acquire.</p>

<p><code>lazy&lt;T&gt;</code> has similar semantics to <code>std::lazy&lt;T&gt;</code>, which is being standardised. See
<a href="https://wg21.link/P1056">https://wg21.link/P1056</a> <em>Add lazy coroutine (coroutine task) type</em>.</p>

<p>The <code>Executor</code> template parameter is purely for compatibility with third party software
such as <a href="https://think-async.com/Asio/">ASIO</a>, and this awaitable can be directly used
by ASIO.</p>

<p>Example of use (must be called from within a coroutinised function):</p>
<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">lazy</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span> <span class="n">func</span><span class="p">(</span><span class="kt">int</span> <span class="n">x</span><span class="p">)</span>
<span class="p">{</span>
  <span class="n">co_return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">...</span>
<span class="c1">// Always suspends perhaps causing other coroutines to execute, then resumes.
</span><span class="c1"></span><span class="kt">int</span> <span class="n">r</span> <span class="o">=</span> <span class="n">co_await</span> <span class="n">func</span><span class="p">(</span><span class="mi">5</span><span class="p">);</span>
</code></pre></div>
<p><code>lazy&lt;T&gt;</code> has special semantics if <code>T</code> is a type capable of constructing from
an <code>exception_ptr</code> or <code>error_code</code> &ndash; any exceptions thrown during the function&rsquo;s body
are sent via <code>T</code>, preferably via the error code route if <a href="../../../reference/functions/error_from_exception.html" class="api-reference"><code>error_from_exception(</code></a>
<code>)</code>
successfully matches the exception throw. This means that a
<a href="../../../reference/types/basic_result.html" class="api-reference"><code>basic_result&lt;T, E, NoValuePolicy&gt;</code></a>
 or <a href="../../../reference/types/basic_outcome.html" class="api-reference"><code>basic_outcome&lt;T, EC, EP, NoValuePolicy&gt;</code></a>
 where one of its types is
is compatible will have its <code>.error()</code> or <code>.exception()</code> set.</p>

<p>Note that <code>lazy&lt;T&gt;</code> does not otherwise transport exception throws, and rethrows
any exceptions thrown within the coroutine body through the coroutine machinery.
This does not produce reliable consequences in current C++ compilers. You should
therefore wrap the coroutine body in a <code>try...catch</code> if <code>T</code> is not able to transport
exceptions on its own.</p>

<p><em>Requires</em>: C++ coroutines to be available in your compiler.</p>

<p><em>Namespace</em>: <code>BOOST_OUTCOME_V2_NAMESPACE::awaitables</code></p>

<p><em>Header</em>: <code>&lt;boost/outcome/coroutine_support.hpp&gt;</code></p>


        </div><p><small>Last revised: March 18, 2022 at 14:45:32 UTC</small></p>
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